Hydraulic kicker control piston

ABSTRACT

A pump-motor unit ( 35 ) including a swashplate ( 95 ) having a neutral position (FIG.  3 ) and first and second displaced positions and comprising first ( 87 ) and second ( 89 ) pressure actuators associated with the swashplate. Displacement of the swashplate is proportional to substantially only the difference in fluid pressure between a high pressure source ( 41 ) and a low pressure source ( 39 ). The first pressure actuator includes a first control piston ( 121 ) having a first effective area (A 1 ), and the second pressure actuator includes a second control piston ( 127 ) having a second effective area (A 2 ), the second effective area being at least somewhat greater than the first effective area. If the high and low pressure sources are at substantially the same pressure, the pressure acting on both of the first and second effective areas will displace the swashplate from neutral toward the second displaced position.

BACKGROUND OF THE DISCLOSURE

The present invention relates to hydraulic drive systems of the typeincluding a variable displacement hydrostatic pump or motor unit, andmore particularly, to such a drive system of the type including apump-motor unit which operates as a pump during a portion of the vehicleoperating cycle, and as a motor during another portion of the vehicleoperating cycle. Even more particularly, the present invention relatesto such a hydraulic drive system in which the pump-motor unit is of thetype having a variable swashplate, the direction of displacement andtilt angle of which are varied only by changes in a control pressurecommunicated to a pair of fluid pressure actuators.

Although the hydraulic drive system of the present invention may beutilized in hydraulic drive systems of various types, including suchdrive systems which effectively serve as the primary vehicletransmission during at least most of the vehicle operating cycle, thepresent invention is especially advantageous when used on a hydraulicdrive system which comprises part of a vehicle hydraulic regenerativebraking system, and will be described in connection therewith. However,it should be understood that the present invention is not limited to useonly in connection with a vehicle drive system, but could be used withvarious types of stationary and/or industrial equipment. Therefore, asused hereinafter and in the appended claims, references to receivingtorque from a “drive line” or transmitting drive torque to such a “driveline”, will be understood to mean and include any sort of torquetransmitting drive line, whether or not such drive line is part of avehicle drive system, or part of some other, non-vehicle type of drivesystem.

In a vehicle hydraulic drive system of the type having regenerativebraking capability, the drive system includes, in addition to thepump-motor unit referenced previously, a high pressure accumulator and alow pressure accumulator, and the appropriate control valving to controlthe flow of fluid, especially between the high pressure accumulator andthe pump-motor unit. In the hydraulic drive system of the presentinvention, the swashplate of the pump-motor unit is tilted in a firstdisplacement orientation when the pump-motor unit is operating in thepumping mode (when the pump-motor unit is receiving torque from thedrive line) and then the swashplate moves “over-center” and is displacedin a second displacement orientation when the pump-motor unit isoperating in the motoring mode (when the pump-motor unit is transmittingtorque to the drive line).

Furthermore, as is now well known to those skilled in the art of suchdrive systems, the “control pressure” used to drive the swashplate ofthe pump-motor unit toward either its first displacement orientation orits second displacement orientation is typically provided by the highpressure accumulator. Disposed between the high pressure accumulator andthe pump-motor unit is a main control valve means which wouldcommunicate control pressure to one of the fluid pressure actuators ofthe swashplate, while communicating the other fluid pressure actuator ofthe swashplate to the low pressure side of the system. Such anarrangement differs from the typical variable displacement axial pistonpump which receives its control pressure from a charge pump driven offof the same input shaft which drives the main axial piston pump rotatinggroup (cylinder barrel and pistons).

Another difference between the pump-motor unit of the type with whichthe present invention is utilized and the conventional, variabledisplacement axial piston pump relates to the centering of the fluidpressure actuators and the swashplate. In the conventional axial pistonpump, each of the fluid pressure actuators is typically spring-centered,but in the pump-motor unit of the invention, the actuators would mostlikely not include centering or return springs. There are severalreasons for omitting the conventional centering springs from thepump-motor unit of the invention. The conventional centering springs arelarge (thus complicating the packaging) and are expensive, and require asubstantial amount of hydraulic energy to overcome, in order to move theswashplate from the neutral position to a displaced position. Finally,the presence of large, high force centering springs in the pump-motorunit of the invention would substantially increase the response time forthe unit to transition between its various modes.

Therefore, the displacement of the swashplate is a function of (i.e.,proportional to) substantially only the pressure differential betweenthe high side of the system (in the subject embodiment, the highpressure accumulator) and the low pressure side of the system (such as alow pressure accumulator). It will be understood by those skilled in theart that, although reference has been made herein to the low pressureside of the system comprising a low pressure accumulator, such is not anessential feature of the invention. All that is essential is that therebe a “low pressure source”, which could be a low pressure accumulator,but could also merely be the case drain region of the pump-motor unit,or a reservoir (whether or not pressurized). All that is essential isthat there be a source of pressure at least slightly greater thanatmospheric pressure, for reasons which will become apparentsubsequently.

Although the hydraulic drive system of the type described above hasproven to be very satisfactory in operation, it has been observed duringthe development of a commercial embodiment of the system that, in theabsence of the present invention, there is at least one operatingcondition under which the pump-motor unit may not operate in the mannerdesired. In the event of leakage from the high pressure side of thesystem (or if there is an extended “idle” period), thus permitting thehigh pressure accumulator to leak down and be at the same pressure asthe low pressure accumulator, then both of the fluid pressure actuatorsof the pump-motor unit would be subjected to the same, low pressure.Under the condition just described, the pump swashplate would move to acentered (neutral, zero displacement) position. When the system logicwould subsequently command displacement of the swashplate, so that thepump-motor unit would operate in either the pumping mode or the motoringmode, there would be no pressure differential to move the swashplatefrom the neutral position toward the appropriate, displaced position.Thus, the pump-motor unit, in the absence of the invention, wouldthereafter be unable either to convert input torque from the drive lineinto stored pressure in the high pressure accumulator, if that is whatwas commanded, or to transmit stored pressure into torque to betransmitted to the drive line, if that is what was commanded.

BRIEF SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide animproved hydraulic drive system having a pump-motor unit of the typedescribed, which overcomes the problem of the swashplate of thepump-motor unit being unable to achieve displacement when the highpressure side of the system leaks down.

It is a more specific object of the present invention to provide such animproved hydraulic drive system, and a pump-motor unit for use therein,in which, even with the high pressure and low pressure sides of thesystem at the same pressure, the fluid pressure actuators of theswashplate are able to move the swashplate to a displaced position.

The above and other objects of the invention are accomplished by theprovision of a hydraulic drive system including a hydrostatic pump-motorunit operable, in a pumping mode, receive torque from a drive line, andoperable, in a motoring mode, to transmit drive torque to the driveline. A high pressure accumulator is in fluid communication with a firstport of the pump-motor unit, and a low pressure source is in fluidcommunication with a second port of the pump-motor unit. The pump-motorunit includes a swashplate having a neutral position and first andsecond displaced positions oppositely disposed about the neutralposition. The pump-motor unit further comprises first and second fluidpressure actuators operably associated with the swashplate to displacethe swashplate toward the first and second displaced positions,respectively. A main control means is in fluid communication with thefirst and second fluid pressure actuators, and with the high pressureaccumulator and with said low pressure source, whereby the displacementof the swashplate is proportional to substantially only the differencein fluid pressure between the high pressure accumulator and the lowpressure source. The first fluid pressure actuator includes a housingportion defining a piston bore and a first control piston disposedwithin the first piston bore, the first control piston having a firsteffective area in fluid communication with the main control means.

The improved hydraulic drive system is characterized by the second fluidpressure actuator including a housing portion defining a second pistonbore and a second control piston disposed within the second piston bore,the second control piston having a second effective area in fluidcommunication with the main control means. The second effective area isat least somewhat greater than the first effective area whereby, in theevent of the high pressure accumulator and the low pressure source beingat substantially the same pressure, the control pressure acting on bothof the first and second effective areas will displace the swashplatefrom the neutral position toward the second displaced position.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of an entire vehicle drive system of the typewith which the hydraulic drive system of the present invention isespecially well suited.

FIG. 2 is a hydraulic schematic of one embodiment of a hydraulic drivesystem of the type to which the present invention relates, including thepump-motor unit of the present invention.

FIG. 3 is a somewhat schematic representation of the pump-motor unit,and specifically, the swashplate and the fluid pressure actuators fordisplacing the swashplate.

FIG. 4 is an enlarged, axial cross-section of the conventional, firstfluid pressure actuator shown schematically in FIG. 3.

FIG. 5 is an enlarged, axial cross-section of the second fluid pressureactuator, shown schematically in FIG. 3, made in accordance with theteachings of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to the drawings, which are not intended to limit theinvention, FIG. 1 illustrates a vehicle drive system of the type forwhich the hydraulic drive system of the present invention is especiallywell suited. The vehicle system shown schematically in FIG. 1 has fourdrive wheels W, although it should be understood that the presentinvention is not limited to a vehicle having four-wheel drive (or evenone having vehicle drive wheels at all, as mentioned in the Backgroundof the Disclosure). The invention could also be used with a vehiclehaving only two-wheel drive, and in that case, the two drive wheelscould be either rear drive wheels or front drive wheels. Operablyassociated with each of the drive wheels W is a conventional type ofwheel brake B, the details of which form no part of the presentinvention, and the wheel brakes B will be referred to only brieflyhereinafter. Preferably, the wheel brakes B, if such were present, wouldbe part of an overall EHB (electro-hydraulic brake) system, of the typewhich is just now becoming well known to those skilled in the art, andcommercially available.

The vehicle includes a vehicle drive system, generally designated 11,which includes a vehicle engine 13 and a transmission 15. It should beunderstood that the particular type of engine 13 and transmission 15 andthe construction details thereof, as well as the drive systemarrangement, etc., form no part of the present invention, except to theextent specifically recited in the appended claims, and therefore, willnot be described further herein. Furthermore, the present invention isnot even limited specifically to use with what is normally thought of asan “engine”, and therefore, it will be understood that, within the scopeof the invention, references to an “engine” will mean and include anytype of power source or other prime mover.

Extending rearwardly from the transmission 15 is a drive line, generallydesignated 17. In the subject embodiment, and by way of example only,the drive line 17 includes a forward drive shaft 19, an intermediatedrive shaft (not visible in FIG. 1), a rearward drive shaft 23, aninter-wheel differential 25 and left and right rear axle shafts 27 and29. Those skilled in the art will understand, from a subsequent readingand understanding of the present specification, that the drive line 17has been illustrated and described as comprising the shafts 19 and 23primarily to facilitate understanding of the overall vehicle drivesystem 11, and not by way of limitation.

The drive system 11, in the subject embodiment, also includes left andright forward axle shafts 31 and 33, respectively. Referring stillprimarily to FIG. 1, in addition to the “mechanical” elements alreadydescribed and which are fairly conventional, the drive system 11 alsoincludes a hydrostatic pump-motor unit, generally designated 35, anddisposed forwardly of the pump-motor unit 35 is a valve manifold 37.Attached to a forward portion of the valve manifold 37 is a low pressureaccumulator 39, and attached to a rear portion of the valve manifold 37is a high pressure accumulator 41, although the particular arrangementcould be reversed, or changed, or rearranged in some other manner. Itshould also be understood that the particular design and details of thevalve manifold 37 (except to the extent noted hereinafter) and theaccumulators 39 and 41 are not essential features of the presentinvention, and therefore, the construction details of each is notillustrated or described herein. Instead, the general function andoperation of each will be described briefly, in connection with thesystem schematic of FIG. 2, but then only to the extent necessary todescribe the several operating modes of the hydraulic drive system as“environment” for the explanation of the pump-motor unit 35, and theswashplate controls therefor, of the present invention.

Referring still primarily to FIG. 1, the pump-motor unit 35 will bedescribed in slightly more detail, to facilitate an understanding of theoverall hydraulic drive system shown in FIG. 1. The pump-motor unit 35includes a clutch assembly portion, generally designated 43, and apump-motor portion, generally designated 45. It may be seen that theintermediate drive shaft extends completely through the hydrostaticpump-motor unit 35 and would preferably have, at its forward end, auniversal joint coupling (not shown herein), for connection to theforward drive shaft 19. Similarly, the intermediate drive shaft wouldpreferably have, at its rearward end, a universal joint coupling (alsonot shown herein), for connection to the rearward drive shaft 23,although, within the scope of the invention, the particular arrangementshown and described could be reversed or changed in some other manner.Thus, the pump-motor unit 35 may either receive drive torque from thedrive line or transmit drive torque to the drive line, depending uponwhether the pump-motor unit 35 is in the pumping mode or the motoringmode, respectively.

Referring now primarily to FIG. 2, it should be understood that, otherthan the pump-motor unit 35 and the two accumulators 39 and 41,everything else shown in the hydraulic schematic of FIG. 2 wouldtypically be included within the valve manifold 37, seen in FIG. 1 orattached to the valve manifold 37. It should also be understood that,whenever the pump-motor unit 35 is in its neutral (zero displacement)condition (which is the case whenever the vehicle is not in adeceleration-acceleration cycle), there is no substantial flow withinthe hydraulic system shown in FIG. 2, between the pump-motor unit 35 andthe two accumulators 39 and 41. However, as is well known to thoseskilled in the art of such systems, because of the pre-charge on each ofthe accumulators 39 and 41, as will be discussed in greater detailsubsequently, the system is intended to remain “pressurized”, even whilethe pump-motor unit 35 is in its neutral (zero displacement) condition.

The hydraulic system (as shown in FIG. 2), which is included within thevalve manifold 37, includes a mode control valve 81, and operablyassociated therewith, a step-orifice control valve 83 and asolenoid-type mode pilot valve 85. The function and operation of thevalves 81, 83 and 85 will be described in slightly greater detailsubsequently, although what will be said hereinafter about the valves81, 83 and 85 will be primarily by way of illustration and enablement ofthe present invention, and not at all by way of limitation of thepresent invention. The construction and operation of the mode controlvalve 81, and the other valves 83 and 85, is illustrated and describedin greater detail in co-pending application U.S. Ser. No. 10/832,967,filed Apr. 27, 2004 in the name of Rodney V. Singh, for a “HydraulicDrive System and Improved Control Valve Assembly Therefor”.

The pump-motor unit 35 is of the variable displacement type, andtherefore, includes some sort of displacement-varying means, such as apair of fluid pressure servo actuators of the type shown in FIG. 2 anddesignated 87 and 89. The servo actuators 87 and 89 are connected,hydraulically, to the outlets of a typical electro-hydraulic controller91 (referred to hereinafter in the appended claims as a “main controlmeans” for the actuators 87 and 89). The function of the controller 91is to communicate pressurized fluid from a conduit 93 to one of theservo actuators 87 or 89, as appropriate, to achieve the desired “mode”,i.e., the desired angle and displacement of a swashplate 95, all ofwhich is generally well known to those skilled in the pump and motorart, and especially the axial piston pump art. Those skilled in the artof hydraulic drive systems of the type to which the invention relateswill understand that, like typical HST systems, there can be mechanicalfeedback from the swashplate 95 of the pump-motor unit 35 to thecontroller 91 (not shown in FIG. 2). Preferably, however, feedback tothe controller 91 is achieved electronically, including even theindication of the position of the swashplate 95. It should be understoodthat any type of feedback arrangement (or none at all) is within thescope of the present invention.

Disposed in series between the high pressure accumulator 41 and theelectro-hydraulic controller 91 is an isolation valve 97 which, as shownin FIG. 2, is preferably a poppet-type valve which is solenoid operated.Whenever the hydraulic drive system 11 is operating, the isolation valve97 is “ON”, i.e., high pressure (“control pressure”) is freelycommunicated from the high pressure accumulator 41 to the controller 91.Whenever the hydraulic drive system 11 is “OFF”, the isolation valve 97is spring biased to the position shown in FIG. 2 in which it keeps thepump-motor unit 35 and the controller 91 “isolated” hydraulically fromthe high pressure accumulator 41, so that the accumulator 41 does not“leak down” through the controller 91, while the system is notoperating. References to the drive system being “OFF” will be understoodto mean and include both that portion of the vehicle operating cyclewhen the vehicle is not in a deceleration-acceleration cycle, as well asthose times when the vehicle is not operating at all (engine “off”conditions).

Referring still primarily to FIG. 2, the drive system 11 includes abypass valve assembly, generally designated 99, which may also bereferred to as an “unloading” valve or as a “dump” valve, as those termsare well understood in the valve art. Thus, the bypass valve assembly 99will “unload” the pump-motor unit 35 whenever the engine is “off”, sothat there is no unintended torque transmitted to the drive line 17. Asis well known to those skilled in the art of hydraulic circuits, thebypass valve assembly 99 would typically be included in such a circuitto “unload” the pump-motor unit 35. It is believed to be within theability of those skilled in the art to determine the specific design andoperation of a particular sub-system, such as the bypass valve assembly99.

The hydraulic drive system 11 also includes a relief valve, generallydesignated 101 which, as is shown in FIG. 2, is spring biased to aclosed position. An inlet of the relief valve 101 is in communicationwith a conduit 103, which interconnects the inlet with the port of thehigh pressure accumulator 41 and with the inlet of the mode controlvalve 81. Whenever the pressure in the conduit 103 exceeds apredetermined maximum, the relief valve 101 is biased (“downward” inFIG. 2) to a position which permits communication from the conduit 103to a conduit 105 (which may be considered the “low pressure” side of thesystem, as will become more apparent subsequently). Finally, referringstill to FIG. 2, the hydraulic drive system 11 includes a filtercircuit, generally designated 107 which forms no part of the presentinvention, and therefore, will not be described further herein.

Referring still to FIG. 2, it may be seen that the pump-motor unit 35includes a port designated A which is connected by means of a conduit109 to the mode control valve 81. The unit 35 also includes a portdesignated B which, by means of a conduit 111 is in fluid communicationwith the filter circuit 107, and also with the conduit 105, such thatthe conduits 105 and 111 comprise the “low pressure” side of the system,as was mentioned previously. As will be seen from the subsequentdescription, when the pump-motor unit 35 is in the pumping mode, theport A is the outlet port (see arrows in pump symbol in FIG. 2), andwhen the unit 35 is in the motoring mode, the port A is the pressurizedinlet port and the port B is the exhaust, outlet port.

Referring still primarily to FIG. 2, the general operation of thehydraulic drive system 11 will be described briefly. As was mentionedpreviously, when the vehicle is neither decelerating or accelerating,the pump-motor unit 35 (pump-motor portion 45 of FIG. 1) is de-clutched,by means of the clutch assembly portion 43, from the intermediate driveshaft, and the overall vehicle drive system shown in FIG. 1 operates inthe same manner as if the hydraulic drive system 11 were not present.

When the vehicle operator begins to perform a braking operation, oneresult is that the clutch assembly portion 43 is actuated, such that thepump-motor unit 35 is now clutched to the drive line 17, and anappropriate command is provided to the electro-hydraulic controller 91(if the unit 35 had been in the motoring mode), displacing theswashplate 95 in a direction such that the rotation of the drive line 17(with the vehicle moving in a forward direction) causes the pump-motorunit 35 to pump pressurized fluid from the port A to the conduit 109. Asis now well known to those skilled in the art of hydraulic regenerativebraking systems, the displacement of the swashplate 95 (and therefore,the fluid output per rotation of the drive line 17) is typicallyproportional to the extent to which the vehicle operator depresses thebrake pedal. It is now known to those skilled in the art how to set thedisplacement of the swashplate 95 proportional to the brake torqueapplied by the operator, or proportional to the displacement of thebrake pedal, although the particular means, or criteria, selected forsetting the displacement of the swashplate 95 is not essential to thepresent invention.

With the pump-motor unit 35 in the pumping mode, pressurized fluidcommunicated through the conduit 109 unseats a poppet member 113 in themode control valve 81, such that the pressurized fluid flows into theconduit 103, and from there, pressurizes the high pressure accumulator41. In the subject embodiment, and by way of example only, the highpressure accumulator 41 is of the gas-charge type. A hydraulic pressureis necessarily maintained such that a minimum amount of oil is alwaysretained in the high pressure accumulator 41 (such that there is alwaysintended to be a minimum pressure charge in both of the conduits 93 and103). At the end of a typical deceleration cycle, the high pressureaccumulator 41 is charged up to nearly the maximum system pressure,typically about 5000 psi.

At the completion of the deceleration portion of the braking cycle, whenthe vehicle operator releases the brake pedal and thereafter, begins todepress the accelerator, an appropriate signal is communicated to theelectro-hydraulic controller 91 which commands the pump-motor unit 35 totransition from the pumping mode (described previously), to the motoringmode. In the motoring mode, the swashplate 95 is disposed at aninclination opposite that which existed when the unit was in the pumpingmode (i.e., the swashplate 95 goes “over-center”). When the pump-motorunit 35 is in the motoring mode, the swashplate 95 is displaced suchthat flow through the pump-motor unit 35 (from port A to port B) willcause the pump-motor unit 35 to transmit torque to the drive line 17,tending to drive the drive line 17 in a direction corresponding toforward movement of the vehicle. In the subject embodiment, and by wayof example only, the mode control valve 81 is constructed such thatpressurized fluid can always flow from the conduit 109 to the conduit103 (i.e., the pumping mode). However, only when the mode pilot valve 85receives an appropriate input signal to its solenoid is there anappropriate pilot signal 115 which assists in the opening of the poppetmember 113, to permit relatively unrestricted flow of high pressurefluid from the accumulator 41 through the conduit 103 and then throughthe conduit 109 to the port A of the pump-motor unit 35.

In the subject embodiment, and by way of example only, the low pressureaccumulator 39 is also of the gas-charge type, and always maintains aminimum inlet charge pressure at the pump-motor inlet port B of about 50psi. This is true even toward the end of the deceleration portion of thecycle, after the unit 35 has pumped up the high pressure accumulator 41.After the completion of the acceleration portion of the cycle, when thelow pressure accumulator 39 contains almost all of the oil, the pressurein the low pressure accumulator 39 rises to about 150 psi, in thesubject embodiment, and by way of example only.

Referring now primarily to FIG. 3, the pump-motor unit 35 is shown insomewhat greater structural detail than in the schematic view of FIG. 2.In the schematic view of FIG. 2 (and in conventional axial pistonpractice), the servo actuators 87 and 89 are shown as being identical.However, and as was described somewhat in the Background of theDisclosure, there is one operating condition under which theconventional servo actuator arrangement, not including the presentinvention, is not fully satisfactory. If, especially after the vehicleis not operated for a period of time, there is a leak-down from the highpressure accumulator 41 through, for example, the controller 91, theremay be a condition in which the entire hydraulic drive system shown inFIG. 2 has “equalized” at low pressure (perhaps equal to the case drainpressure). If the described condition occurs, there will thereafter beno pressure differential available (from the “high” side of the systemto the “low” side) to achieve the desired stroking of the servoactuators.

Therefore, in accordance with one aspect of the present invention, andas may be seen in FIGS. 3 and 4, the servo actuator 87 (“fluid pressureactuator”) is of conventional construction. Thus, there is a housingportion 117 (preferably separate from the main pump housing, butpossibly built into the end cover, etc.) which defines a first pistonbore 119, and reciprocably disposed therein is a first control piston121, having an effective area Al in fluid communication with theelectro-hydraulic controller 91. It should be understood that thepresent invention is not limited to any particular type or configurationof servo actuator, except as is specifically otherwise recited in theappended claims. Therefore, and by way of example only, the firstcontrol piston 121 includes a “slipper” 123, the function of which is toride on the surface of swashplate 95 (see FIG. 3), in a manner wellknown to those skilled in the art, thus transmitting the axial positionof the first control piston 121 to the swashplate 95.

In accordance with a primary aspect of the present invention, and as maybest be seen in FIG. 5, the other servo actuator 89 is not of the sameconfiguration as the servo actuator 87, the differences therebetween tobe explained subsequently. It should be understood that, in accordancewith the invention, there may be a number of parts and aspects of theservo actuators which are the same or similar, but there is one primarydistinction. Thus, the housing portion 117 defines a second bore 125,which is at least somewhat larger than the first piston bore 119.Disposed within the second piston bore 125 is a second control piston127, or more accurately, a second control piston assembly. The secondcontrol piston 127 includes a relatively smaller piston portion 129which includes, in the subject embodiment, a slipper 131, which may beof the same construction and serve the same purpose as the slipper 123shown in FIG. 4.

The second control piston 127 also includes a relatively larger pistonportion 133 which is preferably attached, in any suitable manner, to thesmaller piston portion 129, such that the piston portions 129 and 133reciprocate together as a unit within the second piston bore 125. Inaccordance with the invention, the larger piston portion 133 has aneffective area A2 in fluid communication with the electro-hydrauliccontroller 91, the effective area A2 being at least somewhat greaterthan the effective area Al of the first control piston 121, for reasonsto be described subsequently. The housing portion 117 defines a radialpassage 135 in open communication with a forward portion 137 of thesecond piston bore 125, i.e., the portion disposed between the bore 125and the larger piston portion 133. In the subject embodiment, and by wayof example only, there is an axial vent passage 139 having its forwardend (left end in FIG. 5) in communication with the radial passage 135,and its rearward end open to the atmosphere (14.7 psi., 1.0 bar), forreasons to be described in greater detail subsequently.

In operation, when normal control pressure is communicated to either thefirst control piston 121 or to the second control piston 127, theswashplate 95 is moved in the desired direction (tilt direction), and tothe desired extent (tilt angle), in a manner well known to those skilledin the art of variable displacement axial piston devices. If there is aleak-down condition, the fluid pressures in the conduits 93, 105, 109,and 111 may all be at substantially the same low pressure (e.g.,anywhere from about 50 psi. (3.4 bar) to about 150 psi. (10.2 bar)). Insuch a leak-down condition, and in the conventional, prior art unit,without the present invention, both servo actuators would have been asshown in FIG. 4, in which case, there would be the same low pressureacting on both control pistons (which would have had the same effectiveareas), and the swashplate of the pump-motor unit would have remained inwhatever was the last position of the swashplate, with no readilyavailable means to displace the swashplate to whatever is now itsdesired position, especially if the swashplate was already in itsneutral condition.

With the present invention, the low pressure (during such a leak-downcondition) would be exerting a force over the effective area Al of thecontrol piston 121, but with the same low pressure acting on both endsof the control piston 121, so that it is effectively balanced. At thesame time, the same low pressure is also exerting a force over theeffective area A2 of the control piston 127, but as a result of thesomewhat greater area of the effective area A2, relative to Al, and thefact that the forward portion 137 is vented, there is a net forcetending to move the control piston 127 to the left in FIG. 5, and tomove the swashplate 95 in the “opposite” displaced direction. Thus, theforward portion 137 is vented, by means of the passages 135 and 139, tofacilitate movement of the second control piston 127 to the left in FIG.5, without the resistance to movement which would be present if fluidwere trapped in the forward portion 137. It may now be understood why itis important for the “source of low pressure” (such as the low pressureaccumulator 39) to provide a fluid pressure greater than atmospheric.With the chamber 137 vented to atmosphere, even the relatively “low”pressure is sufficient (if the chamber 137 is vented to atmosphere) tocreate a force imbalance on the second control piston 127 and move theswashplate 95 into at least a slightly displaced condition.

It is important to note that, with substantially the entire hydraulicsystem at low pressure (case, etc.), all other portions of theswashplate displacement control mechanism which “see” the low pressureare the same (i.e., have the same effective area). Therefore, and by wayof example only, the portion of the smaller piston portion 129 whichextends out of the housing portion 117 present exactly the same area asdoes the first control piston 121. Thus, and in accordance with animportant aspect of the invention, the only substantial differencebetween the servo actuators 87 and 89, which results in a differentforce on the swashplate 95, is the effective area A2 being greater thanthe effective area A1.

The invention has been described in great detail in the foregoingspecification, and it is believed that various alterations andmodifications of the invention will become apparent to those skilled inthe art from a reading and understanding of the specification. It isintended that all such alterations and modifications are included in theinvention, insofar as they come within the scope of the appended claims.

1. A hydraulic drive system including a hydrostatic pump-motor unitoperable, in a pumping mode, to receive torque from a drive line, andoperable, in a motoring mode, to transmit drive torque to said driveline; a high pressure accumulator in fluid communication with a firstport of said pump-motor unit, and a low pressure source in fluidcommunication with a second port of said pump-motor unit; saidpump-motor unit including a swashplate having a neutral position andfirst and second displaced positions oppositely disposed about saidneutral position; said pump-motor unit further comprising first andsecond fluid pressure actuators operably associated with said swashplateto displace said swashplate from said neutral position toward said firstand second displaced positions, respectively; main control means influid communication with said first and second fluid pressure actuators,and with said high pressure accumulator and with said low pressuresource, whereby the displacement of said swashplate is proportional tosubstantially only the difference in fluid pressure between said highpressure accumulator and said low pressure source; said first fluidpressure actuator includes a housing portion defining a first pistonbore and a first control piston disposed within said first piston bore,said first control piston having a first effective area in fluidcommunication with said main control means; characterized by: (a) saidsecond fluid pressure actuator includes a housing portion defining asecond piston bore and a second control piston disposed within saidsecond piston bore, said second control piston having a second effectivearea in fluid communication with said main control means; and (b) saidsecond effective area being at least somewhat greater than said firsteffective area whereby, in the event of said high pressure accumulatorand low pressure source being at substantially the same pressure, saidpressure acting on both of said first and said second effective areaswill displace said swashplate from said neutral position toward saidsecond displaced position.
 2. A hydraulic drive system as claimed inclaim 1, characterized by said second piston bore cooperating with saidsecond control piston to define one chamber having said second effectivearea, and being in fluid communication with said main control means, andanother chamber, said another chamber being vented.
 3. A hydraulic drivesystem as claimed in claim 2, characterized by said low pressure sourcecomprising an accumulator of the gas charge type, whereby said lowpressure source and said high pressure accumulator provide, in thepresence of gas pressure in said low pressure accumulator, at least apredetermined minimum control pressure, and said another chamber definedby said second control piston is vented to atmospheric pressure.